After battery was replaced things were fine for about 7 months. Then one morning I started the car and the dash lit up like a Christmas tree, with transmission, iDrive, active steering and ARS errors. After two blocks I pulled over, turned it off then started it again and it seemed fine.

The next day the dash lit up again, so I plugged the voltmeter in and watched it while starting. The voltage read 17V when it was first started! Plenty of extra voltage to burn up every computer in the car. As I drove it would sag to about 11V, then eventually stabilize at 14.2V. The voltage regulator was bad. I could have replaced only the VR, but since it was the original alternator I decided to go with a new OE unit.

Changed out the alternator with a brand new Valero for about $285 delivered, and did my water pump, thermostat, vent tube and both belts as long as I had it all apart.

The BMWs are so dependent upon a strong electrical supply that many things can go wrong, one at a time or all at once if the system sees any significant variation in voltage.